The main issue when roads or paths must be involved is visibility. The thrower needs to be able to see vehicles or people who might come into the potential flight path after release. No blind paths or roads that might be in the flight path - ever. It's a judgment call, but if the traffic on the road or path is above a certain amount, it shouldn't be in play for the design. Roads and paths running parallel to the flight path should be avoided and/or the traffic count on them must be even lower than if the path or park road (never a main road) crosses the fairway ideally near the tee versus near the basket.
I like the advice to imagine the worst conceivable throw, then imagine one even worse.
With roads and walking trails, we also have to consider the stupidest possible disc golfer, too.
Sadly, there are a few out there who will fire away without regard to civilians sharing the park. The consequences of hitting them with a disc could extend far beyond the thrower himself, to the continued existence of the course. And I shudder to think of places where a disc could hit a car on a street with enough traffic to result in an accident.
It is a judgement call, and of course there are places where it's an acceptable compromise. But I've seen a few holes where better judgement could have been used.